1. Field of the Invention
This invention relates to a fluid pressure brake control apparatus for a vehicle which is operable to control the braking slip and the driving slip of the wheels.
2. Description of the Prior Art
For example, the Japanese Patent Official Gazzette No. 94861/1985 discloses the following fluid pressure brake control apparatus: An electro-magnetic three-port three-position change-over valve as a fluid pressure control valve is arranged between a master cylinder and a wheel cylinder. Further, an electro-magnetic three-port two-position change-over valve as a valve apparatus is arranged between the above electro-magnetic three-port three-position change-over valve and the master cylinder, and it is changed over to the position in which the master cylinder side and the wheel cylinder side are cut off from each other, and the wheel cylinder side is made to communicate with a fluid pressure source or a pressure supply source, in the driving-slip control operation.
However, when the above electro-magnetic three-port two-position change-over valve is changed over, the fluid pressure source or pressure supply source is made instantaneously to communicate with the master cylinder. There is the possibility that the high pressure fluid from the fluid pressure source is applied to the piston seal portion in the master cylinder and the latter is damaged.
The Japanese Patent Official Gazzette No. 102367/1986 discloses the following fluid pressure brake control apparatus:
A fluid pressure control valve is arranged between a master cylinder and a wheel cylinder, and it is connected through a valve apparatus to a fluid pressure supply source consisting of a fluid pressure pump and a hydraulic accumulator.
In the driving-slip control operation, the wheel cylinder side and the master cylinder side are cut off from each other by action of the fluid pressure control valve, and the side of the fluid pressure supply source is made to communicate with the wheel cylinder side by action of the valve apparatus.
In the braking slip (anti-skid) control operation, the valve apparatus is changed over to the other position in which the side of the hydraulic accumulator and the side of the fluid pressure control valve are cut off from each other and an open hydraulic reservoir is made to communicate with the fluid pressure control valve. In this apparatus, the fluid pressure control valve consists of two cut-off valve members. The brake pressure is increased, decreased and maintained at constant by actions of the two cut-off valve members. Since the hydraulic reservoir is open to the atmosphere and so the brake conduit forms the open loop, the pressurized brake fluid must be supplied into wheel cylinder from the master cylinder, when the brake fluid pressure should be again increased after the end of the decrease control of the brake fluid pressure. In that case, the forward movement length of the piston in the master cylinder becomes very large, and under certain circumstances, it reaches the full stroke at which the brake pressure cannot be increased more.